This invention relates to a servo device for a disc player.
A disc player for playing data record discs such as video discs and digital audio discs (hereinafter referred to merely as "discs", when applicable) comprises, essentially, a tracking servo system for allowing the data reading spot of the pickup (hereinafter referred to as "a pickup spot", when applicable) to follow the record track whether or not the disc is eccentric and a focus servo system for focusing the pickup spot on the data recorded surface of the disc even when the disc is warped.
Recently, a digital servo has been employed in which an error signal formed according to the output signal of a pickup adapted to read record data from a disc is subjected to A/D (analog-to-digital) conversion so as to be digitally processed.
For instance, in the tracking servo system, when the servo loop is closed, the error components are attributed mainly to the eccentricity of the disc, and therefore the error signal level is small as shown in the part (A) of FIG. 4. When the servo loop is open, as in a search or track jump mode, the pickup spot is moved radially on the disc and the error signal level is much larger than that in the case where the servo loop is closed. The error signal in the open loop condition is shown in the part (B) of FIG. 4.
When error signals of such differing magnitudes are applied to the same A/D converter in a digital servo loop, it becomes clear that the A/D converter will not be effective for the small error signals if set to accommodate the large error signals. For example, if the step size of the converter is selected to accommodate the large range of the open loop error signal, that step size will be much too large to faithfully convert the smaller range error signal when the servo loop is closed.